By mechanical pulps are understood pulps produced primarily by mechanical processing with or without auxiliary steps of a chemical or physical nature. Such pulps include conventional (stone) groundwood and refiner groundwood and pulps produced by an array of chemi-mechanical and thermo-mechanical processes. Such pulps generally have lower bonding properties than chemical pulps and conventionally have been used, e.g. in the making of newsprint, with substantial admixtures of chemical pulp. The tendency has been to reduce more and more such admixtures of chemical pulp and even to use mechanical pulp alone. This trend has been greatly encouraged by the development of the more recent processes of thermo-mechanical and chemi-mechanical pulping and the improvements in the properties of the resulting pulps.
However, the surface properties of paper produced from furnishes of preponderantly mechanical fibers often present problems, particularly accenuated in connection with changes in printing technology, such as the growing acceptance of off-set printing. One such problem is "linting" i.e. the phenomenon of fibers being picked out of the sheet in the process of printing and accumulating on the printing press. Related phenomena are scuffing and dusting and also "picking" in both, wet and dry webs; and in most cases where the term "linting" is used, it is meant to cover all these related phenomena. A high linting propensity of the paper or newsprint will be a nuisance to the printer and in certain cases may cause the paper to be rejected by the customer.
The linting propensity and the associated other deficiencies of the paper are essentially a surface phenomenon and are due to the relatively weak bonding of and between certain fibers on or close to the surface of the paper. As such, the phenomenon is connected in some way with the type of paper machine, the condition of forming, draining and so on. But on any given machine there will still be differences between paper obtained at different times and these differences are accounted for primarily by the pulps used.
It has been proposed to reduce the linting of paper by applying adhesive materials to the surface of the paper, however, this procedure adds considerably to the expenses and is not always effective.
It is, of course, known to beat or refine a pulp to lower its freeness and develop strength and bonding properties, however, mechanical pulps of quite low freeness often still exhibit the phenomenon of linting; moreover, refining the pulp beyond a certain limit will degrade its properties and will also lengthen the drainage time beyond what is acceptable in the papermaking operation. Lower freeness is generally associated with higher average specific surface of the pulp, and generally as would logically be expected leads to a better bonded paper, yet pulps having substantially the same average specific surface often have different linting properties and some pulps of quite low freeness (high average specific surface) produce paper which still lints badly.
It is known to separate from a pulp a "reject" fraction by means of a hydrocyclone and to subject such "rejects" to further refining, but such fractions are generally very small and consist of the relatively unrefined or unfiberized particles.
Malm (U.S. Pat. No. 3,352,745) uses a hydrocyclone to separate a chemical pulp into a springwood fraction and a summerwood fraction, however, Malm's pulp being a chemical unbeaten pulp, the separation is on the basis of coarseness and the method is directed to the achieving of objectives entirely different from those of the present invention. Pesch (U.S. Pat. No. 3,085,927) teaches a process substantially similar to Malm.
Jones (U.S. Pat. Nos. 3,372,879 and 3,411,720) makes a mechanical pulp by means of a refiner having special sinusoidally grooved plates, separates the pulp by screening into an acceptable pulp and non-acceptable wood material, separates the rejected material by hydrocyclone into a coarse fraction and a less coarse fraction, and subjects each fraction to refining, the intensity of refining depending on the coarseness of the fraction. Jones' process is clearly directed to making a mechanical pulp of overall improved strength, by applying to the pulp in total substantially more horse power than is ordinarily done, but it will not selectively reduce linting: on the contrary, by initially separating the pulp on the basis of fiber lengths into about two halves and selecting the shorter fibers as the accepted pulp, Jones from the start includes in his accepted pulp fraction the major part of the fibres which, as will be explained below, from the main source of "lint."
The specific surface as commonly defined expresses the ratio of the surface of the fibers of their weight, e.g. in square meters per gram, and as referred to herein, is obtained using the Robertson & Mason method described in "Specific Surface of Cellulose Fibres by Liquid Permeability Method" Pulp & Paper magazine of Canada, page 103, Dec. 19, 1949.